Various diffusion transfer systems for forming color images have heretofore been disclosed in the art and need not be described in detail in this application. Generally speaking, such systems rely for color image formation upon a differential in mobility or solubility or a dye image-providing material obtained as a function of development of exposed silver halide so as to provide an imagewise distribution of such material which is more diffusible and which is therefore selectively transferred, at least in part, by diffusion, to a superposed dyeable stratum to impart thereto the desired color transfer image. The differential in mobility or solubility may, for example, be obtained by a chemical action such as a redox reaction or a coupling reaction.
In any of these systems, multicolor images are obtained by employing a film unit containing at least two selectively sensitized silver halide emulsions each having associated therewith a dye image-providing material exhibiting desired spectral absorption characteristics. The most commonly employed elements of this type are the so-called tripack structures employing a blue-, a green- and a red-sensitive silver halide layer having associated therewith, respectively, a yellow, a magenta and a cyan dye image-providing material.
A particularly useful system for forming color images by diffusion transfer is that descrived in U.S. Pat. No. 2,983,606, employing dye developers (dyes which are also silver halide developing agents) as the dye image-providing materials. In such systems, a photosensitive element comprising at least one silver halide layer having a dye developer associated therewith (in the same or in an adjacent layer) is developed by applying an aqueous alkaline processing composition. Exposed and developable silver halide is developed by the dye developer which in turn becomes oxidized to provide an oxidation product which is appreciably less diffusible than the unoxidized dye developer, thereby providing an imagewise distribution of diffusible dye developer in terms of unexposed areas of the silver halide layer, which imagewise distribution is then transferred, at least in part, by diffusion to an image-receiving stratum layer to impart thereto a positive dye transfer image. Multicolor images may be obtained with a photosensitive element having two or more selectively sensitized silver halide emullsions and associated dye developers. A tripack structure of the type described above and in various patents including the aforementioned U.S. Pat. No. 2,983,606 is especially suitable for accurate color recordation of the original subject matter.
The resulting image may be revealed by separation of the image-receiving element from the photosensitive element subsequent ot processing, although processes are heretofore known wherein the aforementioned elements are retained in superposition and the image is viewed through a transparent support; for example, as described in Land U.S. Pat. Nos. 3,415,644; 3,415,645; and 3,415,646. Of course, when the image-receiving element is stripped from the remainder off the film unit, the processing composition should not stick to the image-receiving layer in sizable amounts. However, in many instances when thickening agents such as a hydroxyalkyl cellulose either, e.g. hydroxyethyl cellulose, are used in the processing compositions in sufficient concentrations to produce processing compositions of a desired high viscosity, the composition has been found to adhere to an undesirable degree to the receiving element.
Image-receiving elements employed in diffusion transfer color processes typically comprise a plurality of layers coated on one surface of a support, including at least an image-receiving layer adapter to provide a visible image upon transfer to said layer of the diffusible dye image-providing material. Haas U.S. Pat. No. 3,148,061 issued Sept. 8, 1964, discloses image-receiving layers composed of polymers of 4-vinylpyridine which are particularly useful in diffusion transfer processes employing dye developers. As disclosed in Young U.S. Pat. No. 3,388,994 issued June 18, 1968, treatment of the poly-4-vinylpyridine image-receiving layer with ammonia after drying increases the gloss and reduces haze, thereby substantially improving the optical quality of these poly-4-vinylpyridine image-receiving layers.
In addition to the image-receiving layer, the above-described image-receiving elements may further comprise a polymeric acid layer and inert spacer layer, for example, as described in Land U.S. Pat. No. 3,362,819 issued Jan. 9, 1968. The prior art contains several references to diffusion transfer image-receiving elements which also include an external or overcoat layer comprising a hydrophilic colloid to accomplish various purposes. See, for example, Land U.S. Pat. No. 2,759,825 issued Aug. 21, 1956 and Barstow et al, U.S. Pat. No. 3,325,283 issued June 13, 1967, (overcoats to facilitate stripping of the elements); Rogers U.S. Pat. No. 3,295,970 issued Jan. 3, 1967 (an overcoat to increase stability of image to light); and Abbott U.S. Pat. No. 3,698,896 issued Oct. 17, 1972 (an overcoat to provide higher dye densities in color coupler processes). The overcoat layer of the present invention provides an image-receiving element with significantly less haze than those of the prior art as well as providing an efficacious separation of the image-receiving element from the processing composition subsequent to diffusion transfer processing.